The whole likelihood basically hinges on the fact that the outbreak occurred in Wuhan and that the Wuhan Institute of Virology has been working for decades on enhancing coronavirus strains. That's quite strongly circumstantial but it's not evidence. Possible chimerization and furin-cleavage insertion seem a lot more interesting imo but are weighted much lower.
Based on their report [1], most of the likelihood of lab-escape (almost 50x weight) just stems from the fact that the outbreak is in Wuhan. They state that it's because of the proximity to Wuhan Institute of Virology (WIV) and the lab's gain-of-function research - only one of 5 locations world-wide.
That single "bullet-point" re-weights zoonotic origin from 97% to 56% and lab-escape from 1.4% to 42%. Otherwise their final likelihoods would be: "zoonotic" 85.5%, "lab-escape" 8.5%, "bioweapon" 6%.
[1] https://www.rootclaim.com/analysis/what-is-the-source-of-cov...
The initial prior of zoonotic origin simply because that was usually the case in the past is just as circumstantial, but also just as solid in bayesian terms.
>Furin cleavage sites are not common in other related coronaviruses.
However, this claim appears to have been investigated and debunked [1]
>Furin cleavage sites occurred independently for multiple times in the evolution of the coronavirus family, supporting the natural occurring hypothesis of SARS-CoV-2.
[1] https://www.sciencedirect.com/science/article/pii/S187350612...
The Harbin Veterinary Research Institute in Heilongjiang is another BSL-4 lab